1. Field of the Invention
This invention relates to a soft decision signal outputting receiver which outputs, in reception in transmission of a burst signal, a soft decision signal which improves the error correction capability of an error correction decoder to allow data transmission of a high degree of reliability.
2. Description of the Related Art
It is known that, where an error correction code is used, if soft decision decoding wherein a soft decision receive signal is received as an input can be performed, then the correction capability can be improved comparing with hard decision decoding. For example, under white gaussian noise, a coding gain of approximately 2 db in signal-to-noise ratio is obtained by 8-value soft decision decoding comparing with hard decision decoding (refer to, for example, "Soft Decision Technique", Journal of the IECE of Japan, Vol. 67, No. 5, pp. 564-568, 1984).
Meanwhile, in order to transmit data in a channel in which inter-symbol interference occurs, an equalization method for removing an influence of the inter-symbol interference of the channel is required on the receiver side. Particularly, maximum-likelihood sequence estimation (MLSE) is known as an equalization method most suitable to minimize the error rate (for example, John G. Proakis, Digital Communications, second edition, McGraw-Hill, 1989). Accordingly, in order to perform communications with a high degree of accuracy using equalization and error correction decoding simultaneously, it is desirable to employ optimum methods for both of them, that is, to employ maximum-likelihood sequence estimation for the equalization method and employ soft decision decoding for the decoding method.
Various methods have been investigated to obtain a soft decision output from a maximum-likelihood sequence estimator including, for example, "Data signal reception method and apparatus" disclosed in Japanese Patent Laid-Open Application No. Heisei 03-284021, "Method of and apparatus for correcting a Viterbi algorithm" disclosed in Japanese Patent Laid-Open Application No. Heisei 2-288512, "Method of generating a quality coefficient of a binary value obtained by a Viterbi analysis of a signal" disclosed in Japanese Patent Laid-Open Application No. Heisei 4-501945, "SOFT DECISION DECODING WITH CHANNEL EQUALIZATION" (WO91/06165), and J. Hagenauer and P. Hoeher, "A Viterbi Algorithm with Soft-Decision Outputs and its Applications", Proc. GLOBECOM, 1989, Dallas, Tex., pp. 1,680-1,686, Nov. 1989.
However, in such a case that burst transmission is performed in mobile communications or the like, the receive condition (a signal to cochannel interference ratio, a signal to noise ratio or the like) sometimes varies by a great amount in units of a burst or a plurality of symbol times. If a fixed method which does not take a difference in receive condition into consideration is used for a plurality of burst signals whose receive conditions are much different from each other, then reliability information is produced from references which are equivalently different for the individual bursts. Consequently, the conventional method described above is disadvantageous in that the reliability information does not necessarily reflect the true reliability. Further, while a reliability information production method has been proposed, which takes a difference in a part of receive conditions into account, it is disadvantageous in that the processing is complicated.